Process for concentrating acetic acid



Patented Aug. 4, 1936 UNITED STATES PATENT OFFICE PROCESS FORCONCENTRATING I ACETIC ACID Original application February 6, 1931,Serial No.

Divided and this application 'April 12, 1935, Serial No. 16,050

9 Claims. (Cl. 260-122) This invention relates to an improved processfor the concentration of acetic acid from aqueous solutions thereof, andmore particularly to processes involving extraction and distillation. Itis 5 applicable to solutions of practically any percentage or strength,as well as to those which may be contaminated by impurities in solutionor suspension. The process of this invention is furthermore applicableto solutions of acid irre- 10 spective of how such solutions may havebeen formed or obtained, as for example, solutions involvingpyroligneous acid, acetic acid obtained as a waste product inacetylation processes such as the acetylation of cellulose, fermentationacid,

15 etc.

This application is a division of my application 513,989, filed February6, 1931, now U. S. Patent Number 2,028,800 of January 28, 1936.

Among the numerous methods known for the 20 concentration of acetic acidsolutions, two may be mentioned. One isthe method of extracting the acidfrom its aqueous solution with some material which is a solvent for theacid, andwhich is relatively insoluble in Water.

3:. method of removing the water or dehydrating the acid by means of adistillation operation employing some added material which may be calleda withdrawing agent, which forms with water on heating, a constantboiling or azeotropic mix- 30 ture having a boiling point suilicientlylower than that of the acetic acid to permit the rectification of themixture and the resulting removal of water from the acetic acid solutionin the usual operations and equipment familiar in the art ofdistillation.

It is primarily with the first or extraction method that the presentapplication is concerned, although certain aspects of the invention alsomake use of the features of the second or dis- 40 tillation method.

One object of the invention is to provide a generally improved, moreefficient, and more satisiactory process for concentrating ordehydrating acetic acid.

Still another object is the provision of a process of a simple andeifective nature capable of employing dehydrating agents from a classheretofore believed to be unsuitable.

50 Another and very important object is to provide an improved processwhich results in the complete rectification of acetic acid withoutleaving any residue of the withdrawing agent therein.

A further object is the provision of a process 55 which will save asubstantial amount of the heat Another is the i which it has heretoforebeen necessary to use in prior processes of concentrating acetic acid.

A still further and exceptionally important object is the provision of aprocess employing a dehydrating agent which has a marked solubility 5for acetic acid from its solutions so that the agent may be used as anextracting agent for extracting acetic acid from the solution and as awithdrawing agent when treating the extract.

Another object is theprovision of an improved, more efficient, and moresatisfactory continuous process for the concentration of acetic acid.

To these and other ends the invention resides in certain improvementsand combinations of parts, all as will be hereinafter more fullydescribed, the novel features being pointed out in the claims at the end01 the specification.

In the drawings:

Fig. l is a vapor composition curve of a mixture of acetic acid andwater:

Fig. 2 is a vapor composition curve of a mixture of acetic acid andbenzene;

Fig. 3 is a vapor composition curve of a mixture of acetic acid andpropyl acetate;

Fig. 4 is a vapor pressure curve illustrating the vapor pressures ofwater, propylene chloride, and an azeotropic mixture of the two;

Fig. 5 is a vapor pressure curve illustrating water, benzene, and anazeotropic mixture of the two; I Fig. 6 is a similar curve illustratingwater, propyl acetate, and an azeotropic mixture of the two, and

Fig. '7 is a diagrammatic view of a preferred form of apparatus forcarrying out the process.

Similar reference numerals throughout the several views indicate thesame parts.

The general use of a withdrawing agent in concentrating or dehydratingan aqueous solution of acetic acid is explained and set forth in British40 Patent No. 327,444, accepted April 3, 1930, which patent is theresult of a prior invention made jointly by Hans '1. Clarke and thepresent applicant, Donald F. Othmer. Reference is made to said Britishpatent for a disclosure of the general principles underlying the use ofa withdrawing agent (referred to as an auxiliary liquid in said patent),which need not be repeated here. The said patent sets forth eight chiefrequirements for an auxiliary liquid or withdrawing 5o agent for use indistilling water for aqueous acetic acid, which eight requirements stillhold in connection with the present invention, with the exception ofrequirement No. 3. This states that the withdrawing agent should boil ata lower temperature than acetic acid so that it would be readily andcompletely separable from the latter by distillation.

It should be noted that the present invention contemplates the use ofwithdrawing agents which do not boil sufiiciently lower than acetic acidto permit such complete separation readily by distillation, but thisfact is not found to be detrimental because,accordingto the presentinvention, when the process is properly and efllciently carried out,according to the preferred embodiment of this invention, the withdrawingagent does not become mixed with the concentrated acetic acid.

The withdrawing agents which have heretofore been considered suitablefor use in concentrating acetic acid may be divided in general intotwoclasses. The first comprise what might'be termed the low boilingpoint class, such as ethylene dichloride disclosed in the aforesaidBritish Patent No. 327,444, benzol disclosed in United States Patent No.1,722,532, to Maude, and ethyl acetate disclosed in British Patent No.284,588 to I. G. Farbenindustrie Aktiengesellschaft. The second groupcomprises what might be termed the high boiling point class such asbutyl acetate, as disclosed in British Patent No. 298,137 to Dr.Alexander Wacker Gesellschaft.

Withdrawing agents of the low boiling point group have been consideredsuitable because any excess of withdrawing agents could be readilyseparated from the concentrated acetic acid by distillation. Withdrawingagents of the high boiling point group have been unsuitable from thestandpoint of easy separation of excesses by distillation, but it hasbeen possible to use them in some circumstances by following the methoddisclosed in said British Patent No. 298,137. Withdrawing agents boilingbetween the boiling points of the low boiling group and the high boilinggroup have heretofore been considered unsuitable and impracticable foruse because of the difficulty of separating excesses of such withdrawingagents from the concentrated acetic acid.

I have now discovered, however, according to the present invention thatnot only are withdrawing agents boiling in an intermediate range between88 C. and 103 C. suitable for use instead of being unsatisfactory asheretofore believed, but I have further discovered that such withdrawingagents are more suitable and better adapted for the purpose ofconcentrating acetic acid than many of the other withdrawing agentspreviously used.

One of the functions of a dehydrating agent is to form with the water inthe acetic acid solution a mixture which will boil at a constant boilingpoint below the boiling point of either water or the withdrawing agentalone. Such a constant boiling mixture is commonly known in the art asan azeotropic mixture" and will be hereafter referred to by thatdesignation.

Most azeotropic mixtures having water as one of the components, such asthe mixtures hereinafter more fully discussed, are found to follow ingeneral the known principles of steam distillation. It is also foundthat if water be mixed with a liquid which is substantially immiscibletherewith, the respective vapor pressures of the water and the otherliquid are each substantially unaffected by the presence of the other,and the vapor pressure of the mixture is very nearly equal to the sum ofthe individual vapor pressures of the two liquids. Hence a vaporpressure curve of the mixture may be drawn by summing the separate vaporpressure curves of the two component liquids.

Also it is found that the molecular composition of vapors arising fromsuch mixtures is approximately proportional to the respective vaporpressures of the constituents. The higher the vapor pressure of theother liquid mixed with water, the greater will be the number ofmolecules of such other liquid in any given quantity of vapor evolvedfrom the mixture.

The efilciency of a concentration process involving an azeotropicmixture of water and a withdrawing agent depends to a material extent onthe relative proportions of water and withdrawing agent which arevaporized. This depends, in turn, upon the relative molecularpercentages of the water and of the withdrawing agent, which molecularpercentages, as above stated, are approximately proportional 'to therespective vapor pressures of the water and the withdrawing agent at theparticular pressure under which boiling takes place, generally andpreferably atmospheric pressure, although subatmospheric orsuperatmospheric pressures may be used if desired. Since the normalboiling points of liquids are temperatures at which their vaporpressures are the same, i. e. 76 centimeters of mercury, it follows thatthe vapor pressures of different liquids vary in general more or lessinversely with their boiling points. Therefore, a statement of theboiling point of a liquid gives a general rough indication of itsrelative vapor pressure characteristics, the liquids of higher boilingpoint having in general at all temperatures lower vapor pressures thanliquids of lower boiling point.

Hence from the standpoint of efiiciency as determined by the vaporpressure of the withdrawing agent, it is desirable to use withdrawingagents having relatively high boiling points and consequently lowervapor pressures. At the same time, the use of a withdrawing agent havinga high boiling point is accompanied by the disadvantage that such anagent will not lower the boiling point of an azeotropic mixture withwater to a sufiicient extent to permit easy distillation of theazeotropic mixture from the acetic acid, the boiling point of which isapproximately 118 C. Also, a withdrawing agent of high boiling point hasthe further disadvantage that its boiling point is so close to theboiling point of acetic acid that the withdrawing agent is apt tocontaminate the concentrated acid during the distillation process. Thusin selecting withdrawing agents suitable for use, the advantages ofhigher or lower boiling points must be balanced with the correspondingdisadvantages, and suitable agents must be selected which have to aslarge an extent as practicable, the advantages of high boiling pointwithout an impractically large amount of the disadvantages thereof.

By balancing the various factors in the manner outlined above, I havediscovered that some very suitable extracting and withdrawing agents foruse in concentrating aqueous acetic acid solutions, are those which haveboiling points at normal atmospheric pressure of more than 88 C. andless than 103 C. Such dehydrating agents which have boiling pointsbetween 88 C. and 103 C. have in general vapor pressures which, inazeotropic mixtures at or near normal atmospheric pressure, are notmaterially greater than the vapor pressure of the water in theazeotropic mixtures, so that a process employing such withdrawing agentshas relatively high efi'iciency; yet

' tures with water.

the vapor pressures are sufliciently high so that the boiling point ofthe azeotroplc mixture of the agent and water is sufliciently lower thanthe boiling point of acetic acid to permit satisfactory distillation ofthe azeotropic mixture from the acetic acid solution and from pureacetic acid.

Viewed from a slightly diiferent aspect, I find some extracting andwithdrawing agents are those whose vapor pressures, at the temperatureof the boiling point of the azeotropic mixture with water, are not morethan 60% nor less than 40% of the total vapor pressure of the azeotropicmixture. When the vapor pressure of the withdrawing agent is more thanabout 60% of the total vapor pressure of the azeotropic mixture, thenthe molecular percentage of the vapors from the azeotropic mixture issuch that an uneconomically excessive quantity of withdrawing agent mustbe distilled over for each unit quantity of water removed from themixture. Similarly, when the vapor pressure of the withdrawing agent isless than about 40% of the total vapor pressure of the mixture, thevapor pressure of the withdrawing agent is not sufiiciently high toreduce the boiling point of the azeotropic mixture to a sufficientextent to permit easy and satisfactory distillation of the azeotropicmixture from the acetic acid solution and from pure acetic acid. Hencethe vapor pressure limits of 60% and 40% above mentioned have been foundto be very suitable, and the present invention contemplates the use ofextracting and withdrawing agents having a vapor pressure between theselimits.

Viewing the matter from still another aspect, I find that in generalsuitable dehydrating agents are those which form azeotropic mixtureswith water in which the azeotropic mixture has a normal boiling pointbetween 76 C. and 86 C. The three sets of limits above specifiedcorrespond in general with each other. That is, agents whose vaporpressures are within the specified limits of 60% and 40% will generallybe found to have boiling points approximately between the specifiedranges of 88 C. and 103 C., and it will further be found that suchagents form azeotropic mixtures with water which mixtures have boilingpoints approximately between the specified limits of 76 C. and 86 C.

Thus it is the physical or physical-chemical properties rather than thestrictly chemical properties of the agents which are found to be mostimportant according to the present invention. Of course all liquidswhich have boiling points between 88 C. and 103 0. would not be suitablefor the purposes of the present invention, because all of such liquidswould not function as withdrawing agents. A withdrawing or dehydratingagent, as the term is intended in this specification and in theaccompanying claims, may be defined as a liquid which will form aconstant boiling or azeotropic mixture with water. Generally andpreferably, a withdrawing agent is substantially immiscible with water,although some withdrawing agents which are miscible with water areknown. Those withdrawing agents which are substantially immiscible withwater form azeotropic mixtures therewith in which the vapor pressure ofthe mixture at any temperature is approximately the sum of the separatevapor pressures of water and the withdrawing agent.

Thus, by definition, the term withdrawing agent as herein usedautomatically excludes all substances which will not form azeotropicmix- I find that any withdrawing agent having a boiling point between 88C. and

103 C. is suitable and satisfactory when used in the concentrating ofacetic acid according to the present invention, whereas withdrawingagents boiling within this range of temperatures have heretofore beenconsidered unsuitable and impracticable.

As examples of suitable withdrawing agents boiling within thistemperature range at normal atmospheric pressure, I have found thefollowing materials to be-more or less advantageous for the processdescribed, the approximate boiling point under normal atmosphericpressure bteing indicated after the name of each subs ance:

Certain of these substances, such as allyl iodide and normal heptane,form azeotropic mixtures at certain percentages with acetic acid as wellas with water, and consequently have been found useful in concentratingacetic acid only between certain ranges of strength or in concentratingacetic acid where an anhydrous acid mixed with some proportion ofwithdrawing agent may be discharged for use in some further operation.The use of a withdrawing agent which forms an azeotropic mixture withacetic acid is not generally desirable, but may be suitable underspecial circumstances as above mentioned. Benzene, known in the priorart as a withdrawing agent (see Maude Patent No.

1,722,532, above mentioned), forms an azeotropic mixture with aceticacid when the mixture contains about 98% benzene, but this does notprevent its use under some circumstances. Similarly the use of awithdrawing agent within the boiling range specified such as normalheptane, which forms an azeotropic mixture with acetic acid, makes itdifiicult or uneconomical to concentrate dilute aqueous acetic acid bythe preferred process described, but if the amount of water in thestarting solution is only comparatively small, the acid may bedischarged from the still pot in a practically pure condition althoughthe water will also contain substantial amounts of acid.

Other of the agents disclosed, which do not form azeotropic mixtureswith acetic acid, are in general more suitable than those which do formsuch mixtures; It has been found in my extraction process thatexceedingly satisfactory results are obtained by using propyl acetate,ketones, either singly or admixed, and at present it is preferred to useone or another of these substances, which are commonly available atreasonable cost, and especially normal acetate.

Certain substances mentioned above, as for example diethylene dioxide(1,4-dioxan), do not, in their azeotropic mixtures with water, separateinto two layers on standing. Consequently 89.5" C. 90 C. 902 C. 91.6 C.94 C. 96 C. 97 C. 985 C. 99 C.

p py

' when such a substance is used as an extracting and withdrawing agent,it is more diflicult to separate the agent from the water aiter thedistillation. Consequently, agents of this kind may be used to advantageunder some conditions, and their use is within the scope or thisinvention.

To aid in understanding the physical properties of the withdrawingagents and mixtures of the present invention, various curves areillustrated in Figs. 1 to 6 inclusive. Referring to these, Fig. 1 showswhat is called a vapor composition curve of water and acetic acid. Theabscissae of the graph are the percentages of water in the mixture,while the ordinates are the percentages of water vapor in the totalvapor evolved by boiling. The 45 line is a reference line for the sakeof comparison and indicates a percentage of water in the vapor equal tothe percentage of water in the liquid mixture. It will be seen that thevapor composition curve of water and acetic acid diverges only to acomparatively slight extent from the 45 line so that only slightly morewater vapor than acetic acid vapor is given off when a mixture of thetwo is boiled. This indicates that a simple mixture of water and aceticacid cannot he satisfactorily concentrated by distillation, and showswhy it is necessary to use a withdrawing agent in order to concentrateaqueous acetic acid economically.

Fig. 2 illustrates a similar vapor composition curve of benzene andacetic acid in which the percentages indicated refer to benzene. Themain curve is plotted on the main abscissae and the mainordinatesindicated at the bottom and left hand side of the graph,respectively, while the supplementary curve is an enlargement of theupper right hand end of the main curve and is plotted on the enlargedscale abscissae and ordinates indicated at the top and right hand sideof the graph, respectively, as is commonly understood by those familiarwith curves of this kind. The enlarged supplementary curve clearly showsthat the vapor composition curve crosses the 45 line when the benzene isapproximately 98% of the total mixture, and such crossing of the 45 lineindicates that at this point an azeotropic mixture of acetic acid andbenzene is formed. Hence, as already mentioned above, the usefulness ofbenzene, one of the prior art withdrawing agents, is limited.

The graph of Fig. 3 shows a vapor composition curve of acetic acid andnormal propyl acetate, the percentages of propyl acetate in the mixturebeing indicated by the abscissae and the percentages of propyl acetatevapor in the total vapor being shown by the ordinates. This vaporcomposition curve does not cross the 45 line, and thus indicates thatpropyl acetate does not form an azeotropic mixture with acetic acid.Hence, propyl acetate may be satisfactorily used as an extracting andwithdrawing agent to concentrate aqueous acetic acid of any percentageof strength. The closeness of the vapor composition curve to the 45 linedoes indicate, however, that it is difficult to distill an excess ofpropyl acetate from the final concentrated acetic acid in an economicalmanner, and for that reason the concentration is preferably carried onby a continuous process hereinafter described which does not result inan excess of propyl acetate in the acetic acid.

Referring now to Fig. 4 there is shown a vapor pressure curve of waterindicated by the line [0, a vapor pressure curve of propylene chlorideindicated by the line H, and a vapor pressure curve of an azeotropicmixture of water and propylene chloride indicated by the line l2. Thetotal vapor pressure or the mixture at any particular temperature issubstantially the sum of the two separate vapor pressures at thattemperature, as explained above. In this graph, the abscissae aregraduated in degrees centigrade from 50 to 100, as the portions of thecurves below 50 C. are unnecessary for the present purposes. Theordinates are graduated in pressures expressed as centimeters of mercuryfrom to 80. The reference line l3 indicates a pressure of '76centimeters or normal atmospheric pressure.

It will be seen that the curve I2 of the mixture crosses the line l3 ofnormal atmospheric pressure at approximately 78 C., indicating this asthe boiling point of the mixture, which is between the limits of 76 C.and 86 C. above mentioned. 01 the total vapor pressure of '76centimeters of mercury, about 33 centimeters is due to the vaporpressure of the water, as indicated by the bracket it, while theremainder or about 43 centimeters is due to the vapor pressure of thepropylene chloride, as indicated by either of the two brackets l5 whichsubtend an equal distance. Hence it will be seen that the vapor pressureof the propylene chloride at the boiling point of the mixture, is about57% of the total vapor pressure of the mixture, and is between thelimits of 60% and 40% above mentioned.

As the vapor pressures of the components of a mixture of this kind arevery nearly proportionate to the respective molecular percentages, itfollows that of every 76 molecules in the vapor evolved from the liquidmixture, about 33 molecules will be water molecules and about 43molecules will be propylene chloride molecules. By reducing topercentages and multiplying the molecular percentage by the molecularweights, the relative quantities by weight of water and of propylenechloride evolved from boiling an azeotropic mixture of the two can bereadily calculated. When this is done, it is found that of every unit ofweight of the mixture which is distilled, approximately 11% by weight iswater and approximately 89% by we'ght is propylene chloride. In otherwords, for every unit by weight of water which is distilled from anazeotropic mixture of water and propylene chloride, it is necessary todistill eight units by weight of propylene chloride. Multiplying this bythe latent heat of vaporization of propylene chloride, which is 75calories per gram, it is found that 600 calories are necessary todistill the quantity of withdrawing agent needed to carry over each gramof water in the mixture. This is relatively efficient in comparison towithdrawing agents which have been used in the prior art, in which muchgreater proportions of withdrawing agents have ordinarily had to bedistilled, with consequent greater consumption of heat.

For the sake of comparison, Fig. 5 shows a similar set of vapor pressurecurves of water and benzene, one of the prior art withdrawing agents,and of a mixture of the two. The curve is that of water, 2i that ofbenzene, and 22 that of the azeotropic mixture of the two. Line 23indicates normal atmospheric pressure of '76 centimeters, and crossesthe line 22 at about 68, indicating this as the boiling point of themixture. The bracket 24 indicates that of the total vapor pressure of 76centimeters only a relatively small proportion or about 22 centimetersis due to the vapor pressure of the water, while a relatively largeproportion or about 54 centimeters is due to the vapor pressure of thebenzene as indicated by the brackets 25. Hence when using benzeneaccording to the prior art, it is necessary to distill many moremolecules of the withdrawing agent for each given quantity of watermolecules distilled than when using propylene chloride according to thepresent invention. It will be seen that the vapor pressure andconsequently the molecular percentage of the benzene is more than twicethat of the water, and about 71% of the total vapor pressure of themixture. When multiplying the molecular percentages by the molecularweights as above indicated, it is found that for every unit of waterdistilled from an azeotropic mixture of water and benzene, it isnecessary to distill 10.6 units by weight of benzene, and since thelatent heat of vaporization of benzene is 94, it requires almost exactly1000 calories to distill the withdrawing agent necessary to carry overone gram of water from the mixture. This will be seen to be much morewasteful of heat than when propylene chloride is used according to thepresent invention.

Referring now to Fig. 6, there are shown corresponding vapor pressurecurves of water, indicated by the numeral 30, normal propyl acetate,indicated by the numeral 3|, and an azeotropic mixture of water andpropyl acetate, indicated at 32. The reference line 33 indicates normalatmospheric pressure of '76 centimeters, and shows that the boilingpoint of an azeotropic mixture at normal pressure is about 81 C. whichis sufliciently far removed from the boiling point of acetic acid (about118 C.) to make it possible to distill the azeotropic mixture from theacetic acid satisfactorily.

The bracket 34 indicates the vapor pressure of the water at thistemperature, which will be seen to be about 37 centimeters of mercury,while the bracket 35 indicates the vapor pressure of the propyl acetate,amounting to about 39 centimeters. In this instance, the proportion ofvapor pressure or molecular percentage of the propyl acetate to thetotal vapor pressure or molecular percentage is about 51%, even lessthan that of the propylene chloride illustrated in Fig. 4 and much lessthan that of the benzene illustrated in Fig. 5.

Multiplying the molecular percentages by the molecular weights, it isfound that of a unit of weight of vapor, about 14.4% by weight is watervapor and about 85.6% by weight is propyl acetate vapor, or in otherwords, 5.9 units by weight of propyl acetate must be distilled with eachunit of weight of water. Multiplying this by the latent heat ofvaporization of propyl acetate, it is found that 472 calories must beused to distill the amount of withdrawing agent necessary to carry overeach gram of water from the azeotropic mixture. This is a great dealmore eflicient than when benzene is used as the withdrawing agent, andsomewhat more efficient than when propylene chloride is used. Hence Iprefer when practicing the present invention to employ normal propylacetate, although I also contemplate using any other extracting andwithdrawing agent boiling within the temperature range of 88 C. to 103C., as above mentioned, and especially iso-propyl acetate or ketonessingly or mixed with dioxan.

The boiling points, molecular percentages, and other figures given aboveby way of example are those obtained by calculation. In actual practicethe results are found to be slightly different from but quite close tothose calculated and in general it is found that the actual results areto a slight extent even more advantageous and emcient than thecalculated ones.

The various temperatures and vapor pressures above mentioned have beendetermined on the assumption that distillation will take place at normalatmospheric pressure of 76 centimeters of mercury, since this isordinarily preferred. It is possible, however, to carry out the processof the present invention at pressures either above or below normalatmospheric; at these pressures withdrawing agents of the presentinvention will be found still to have approximately the samerelativeefliciencies with respect to withdrawing agents of the priorart.

The use of normal propyl acetate and other I of the agents asdehydrating agents has a further advantage not only because of theirincreased efiiciency over withdrawing agents of the prior art, but alsobecause they exhibit a useful solubility for acetic acid while in someinstances being substantially insoluble in water. Hence propyl acetate,according to the present invention, may be used as an extracting agentin addition to its action in the distillation step. For instance, thepropyl acetate may be brought into contact with the dilute acetic acidto dissolve the acetic acid in the propyl acetate, and then the propylacetate extract with acid and water dissolved therein may be led to astill and distilled.

Referring now to Fig. '7 of the drawings, there is shown a diagrammaticrepresentation of an embodiment of apparatus for carrying out thedistillation step of the present invention. The numeral 40 indicates acolumn still of any suitable and well known construction. For example,the still may be of the type having a multiplicity of plates and theusual bubble caps. It is heated in any suitable manner, such as by thecoil 4| to which steam or other heating fluid is supplied from3 theinlet 42 and discharged from the outlet To aid in controlling theprocess, the still is provided at intervals throughout its height withsuitable temperature indicating devices such as the thermometers 44. I

To start the process the still may be initially charged with the aqueousacetic acid in the form of extract or dilute acid feed to beconcentrated, and with the withdrawing agent selected for'use. Thewithdrawing agent may be supplied through the conduit 45 from thecontainer 46 and/or from the extract. Ordinarily, after the still isinitially charged with withdrawing agent, no further withdrawing agentis introduced except as small amounts may be required from time to timeto make up for losses in the system by leakage. The withdrawing agentwith which the still is originally charged flows through a continuouscircuit and is returned to the still or to the extract as describedhereinafter for use over and over again.

In the manner above described, the withdrawing agent will form anazeotropic mixture with the water in the aqueous acetic acid content ofthe mixture in the column and the boiling point of this azeotropicmixture will be sufflciently 52 and flows to the gravity or othersuitable separator 53. The acetic acid, meanwhile, gradually worksdownwardly in the column and may be removed from the bottom thereof inliquid phase if desired, although it is preferred to remove theconcentrated acetic acid in vapor phase through the outlet 54 leading tothe condenser 55, where it is condensed to liquid phase and dischargedthrough the conduit 56 to any desired point.

The process is preferably carried on continuously once the still hasbeen charged and started, additional aqueous acetic acid being suppliedto the still through the inlet conduit 50 leadin from any suitablesource of supply such as extract and/or dilute acid feed held in thereservoir 81. The aqueous acid supplied through the conduit 50 may be ineither liquid or vapor phase, and the conduit may enter the column at anintermediate point in the height thereof, which point will be determinedin the usual manner well understood by those skilled in the art.

The azeotropic mixture of water and with drawing agent, after enteringthe separator 53, separates into two layers, a lighter layer at the topand a heavier layer at the bottom. A conduit 84 leads from the layer ofwithdrawing agent to the still and/ or extractor so that the withdrawingagent after being separated is returned substantially continuously tothe still or extractor to be used over and over again. In case thewithdrawing and extracting agent employed is propyl acetate which islighter than water, the top layer 65 in the separator 53 will be propylacetate while the bottom layer 66 will be water. Consequently theconduit 64 is connected to the separator 53 near the top thereof, and awater discharge conduit 61 leads from a point near the bottom of theseparator. In case the agent employed is one which is heavier thanwater, however, the relative positions of the layers in the separatorwill be reversed, of course, and the relative positions of the conduits64 and 61 will be correspondingly reversed.

While the separator 53 substantially separates the withdrawing agentfrom the water, there is some slight amount of withdrawing agentdissolved in the water. Frequently the amount of withdrawing agentdissolved in the water is not sufficient to justify recovery thereof,and the conduit 61 may then lead to waste, but if it is desired underany particular economic conditions to recover the dissolved withdrawingagent, then the conduit 67 may be led to any suitable recoveryapparatus, such for example as the auxiliary still 68 supplied withsteam through the inlet 69. In this still, the dissolved withdrawingagent is separated from the water and the vapors of the withdrawingagent pass off from the top of the still through the conduit 10 to acondenser H in which they are condensed and from which they are ledthrough the conduit 12 to the separator 53. Thus the withdrawing agentdissolved in the water layer of the separator is recovered and returnedto the separator, whence it, together with the withdrawing agent firstseparated from the water in the separator, is supplied to the main still40 or an extractor for further use. The water separated from thewithdrawing agent in the auxiliary still 58 may be discharged therefromthrough the discharge conduit 13.

Since substantially all of the withdrawing agent distilled off from thecolumn still through the conduit 50 returns to the column, it followsthat the rate of supply of the withdrawing agent to the column isordinarily not variable, when the proper amount is in the system. Thevariable factors of the process are the rate at which heat is suppliedthrough the conduit 42 and the rate at which acetic acid is suppliedthrough the con- 5 duit 60. These two rates are varied as desired inorder to obtain proper operation of the apparatus.

It is ordinarily desirable in operation to have at all points in thecolumn the correct azeotropic ratio of withdrawing agent to the water inthe acetic acid, even though it is only at the top of the column thatthe vapors are composed wholly of the azeotropic mixture to theexclusion of acetic acid vapor. At successively lower points in thecolumn, where-acetic acid is present in relatively greater and greateramounts and water is present in relatively smaller and smaller amounts,it is nevertheless desirable to maintain the theoretically correctazeotropic ratio between the 0 water and the withdrawing agent. Suchmaintenance of correct ratios throughout the column constitutes theideal case, and in practice it is ordinarily not possible to maintainthis ratio correct at every point, but it may be maintained 25 in ageneral or aproximate way. For example, a branch of the conduit 54 maylead to some lower point of the column at which a small fraction of thereturning withdrawing agent may be supplied.

Heretofore, prior processes of concentrating acetic acid have usuallyemployed excesses oi withdrawing agent, rather than just the rightamountof withdrawing agent as contemplated by the present invention. Hencewithdrawing agent 35 has heretofore been present at most points of thecolumn, the lower portion of the column having no water present but onlya mixture of acetic acid and withdrawing agent.

The present invention contemplates, and it is of great importance, thatthe variable factors (rate of heating and rate of supply of aqueousacetic acid) should be so controlled that there will be no excess ofwithdrawing agent, and that at the point in the column at which the lastof the water disappears, the last of the withdrawing agent will alsodisappear.

By watching the thermometers or other heat indicating devices 44 at thevarious points up and down the column, an operator skilled in the op- 5eration of a column still is enabled to ascertain very closely thecomposition of the mixture at various points in the column, and in thisway he is enabled to control the process to accomplish the desirableresult above set forth, varying the rate 5 of heating and acetic acidsupply as necessary, and adding additional withdrawing agent from timeto time if this is found to be necessary.

When the process is carried on in the above desired manner according tothe present invention, the still pot will contain substantially pure orglacial acetic acid, instead of acid having withdrawing agent mixedtherewith as has usually been the case heretofore. Consequently therewill be no problem of separating the withdrawing agent from thedehydrated acetic acid, and the latter will flow from conduit 56 insubstantially pure form ready for any desired use.

When the withdrawing agent employed in the process is one of thosefalling within the scope 70 of the invention, which are more efiicientin general than the prior art withdrawing agent for the reasonspreviously mentioned, it is found that the volume of vapors distilledfrom the column to obtain a given amount of glacial acetic acid is sarychemical resisting qualities.

According to a modified process contemplated by the present invention,instead of adding a withdrawing agent to the dilute acetic acid, thewithdrawing agent may be formed by chemical reaction in the diluteacetic acid itself, the substance added reac'ting either with the aceticacid or with the water which dilutes it. For example, propyl alcohol maybe added to the dilute acetic acid to form, by employing the properconditions to produce reaction therewith, propyl acetate, which is thepreferred withdrawing agent of the present invention. Hence the presentinvention is intended to include agents formed or added in this manner,provided they fall within the temperature and other ranges specified, aswell as those formed externally and added later to the dilute aceticacid.

The present invention also contemplates the use as an agent, a mixtureof any two or more substances which when mixed will boil or have othercharacteristics within the ranges heretofore specified, regardless ofwhat the boiling points or other characteristics of the individualcomponents before mixture may be.

According to the preferred embodiment of the present application whichis directed primarily to a process involving extraction, instead ofpassing the withdrawing agent from the separator 53 to the still column10, as through the conduit 54, some fraction of it or all may first beled to an extractor of any suitable known type for extracting one liquidby another, in which extractor the withdrawing agent will come intocontact with aqueous acetic acid and extract the acetic acid from thewater. The mixture of withdrawing agent, acetic acid, and the waterwhich is unavoidably present and taken up in varying amounts dependenton the particular agent employed and the concentration of the aceticacid extracted, is then supplied to the column still and rectified inthe manner previously described, that is, to remove whatever water ispresent and leave a concentrated acetic acid free of withdrawing agent.As previously described in detail, this is accomplished by controllingthe various factors such as the rate of. heating, aqueous feed, andquantity of agent.

The bulk of the water which was originally mixed with the acetic acid isdischarged from the extractor saturated with the withdrawing agent, andit may then be introduced through the conduit 6! into the auxiliary unit68 to have the withdrawing agent recovered in the manner previouslydescribed, the water free from withdrawing agent being dischar edthrough the conduit l3.

While certain embodiments of the invention have been disclosed, it is tobe understood that the inventive idea may be carried out in a number ofways. This application is therefore not to be limited to the precisedetails described, but is intended to cover all variations andmodifications thereof falling within the spirit of the invention or thescope or; the appended claims.

What I claim is:

1. The continuous process of concentrating aqueous acetic acid includingthe use of an extracting and withdrawing agent and extraction anddistillation, which comprises feeding a distillation system with aqueousacetic acid and a quantity of an agent having withdrawing and extractingproperties and a boiling point between 88 C. and 103 C. not in excess ofthe right amount to form an azeotropic mixture with the water containedin the aqueous acetic acid, distilling water and agent from the mixture,condensing the distillate, separating distillate into an extracting andwithdrawing agent layer and a water layer, returning-at least a part ofthe extracting and withdrawing agent into extractive contact withaqueous acetic acid undergoing concentration, and subjecting theresultant extract mixture of aqueous acetic acid and agent todistillation in the said system.

2. A process for concentrating aqueous acetic acid includingdistillation and extraction, which comprises subjecting aqueous aceticacid to at least one extraction treatment with a dehydrating agenthaving withdrawing and extracting properties and a boiling point between88 C. and 103 C., subjecting the resultant extract to distillation underconditions wherein the withdrawing and extracting agent is maintained ina quantity not in excess of the right amount to form an n azeotropicmixture with the water contained in cipally of withdrawing andextracting agent and another layer comprised principally of. water,returning at least a portion of the withdrawing and extracting agentlayer to the still, and removing from the still concentrated acetic acidfree of withdrawing and extracting agent.

3. A process for concentrating aqueous acetic acid includingdistillation and extraction, which comprises subjecting aqueous aceticacid to at least one extraction treatment with a dehydrating agenthaving withdrawing and extracting properties and a boiling point between88 C. and 103 0., subjecting the resultant extract to distillation underconditions wherein the withdrawing and extracting agent is maintained ina quantity not in excess of the right amount to form an azeotropicmixture with the water contained in the distillation unit, distillingoil water and dehydrating agent, condensing the distillate, sepmatingthe distillate into a layer comprised principally of withdrawing andextracting agent and another layer comprised principally of water,returning at least a portion of the withdrawing and extracting agentlayer to the distillation system, and removing from the distillationsystem in the vapor phase concentrated acetic acid free of withdrawingand extracting agent.

4. A continuous process for concentrating aqueous acetic acid includingthe use of an extracting and withdrawing agent and distillation, whichcomprises feeding a distillation system with any extract comprisingaqueous acetic acid and an agent having withdrawing and exractingproperties and a boiling point between 88 C. and 103'C., subjecting thematerials to a distillation treatment which includes controlling theheating of the distillation system, the rate of supply of acetic acidand maintaining the withdrawing and extracting agent not in excess ofthe right amount to form an azeotroplc mixture with the water containedin the acetic acid, vaporizing a distillate comprising water Cir andwithdrawing and extracting agent, condensing the distillate, recoveringand returning to the distillation step at least a part of thewithdrawing and extracting agent oi! the condensed distillate.

5. The continuous process of concentrating aqueous acetic acid includingdistillation and extraction, which comprises charging a distillationsystem with aqueous acetic acid, and a quantity of a dehydrating agenthaving withdrawing and extracting properties and a boiling point between88 and 103 C. not in excess 0! the right amount to form an azeotropicmixture with the water contained in the aqueous acetic acid, distillingwater and the dehydrating agent from the mixture, condensing thedistillate, dividing the condensed dehydrating agent containingdistillate into portions, returning a portion or the condensed agent tothe distillation step, passing at least a portion of the agent obtainedfrom the distillate into contact with aqueous acetic acid to extract theacetic acid therefrom, and returning to the distillation step theresultant extract.

6. A process for concentrating aqueous acetic acid includingdistillation and extraction, which comprises subjecting aqueous aceticacid to at least one extraction treatment with propyl acetate,subjecting the resultant extract to distillation under conditionswherein the propyl acetate is maintained in a quantity not in excess ofthe right amount to form an azeotropic mixture with the water containedin the distillation system, distilling off water and dehydrating agent,condensing the distillate, separating the distillate into a layercomprised principally of propyl acetate and another layer comprisedprincipally of water, returning at least a portion of the propyl acetatelayer to the distillation step and withdrawing from the distillationstep concentrated acetic acid free of propyl acetate.

7. The continuous process of concentrating aqueous acetic acid includingthe use of an extracting and withdrawing agent and extraction anddistillation, which comprises substantially continuously feeding adistillation system with aqueous acetic acid in the vapor phase and aquantity of an agent having withdrawing and extracting properties and aboiling point between 88 C. and 103 C. not in excess of the right amountto form an azeotropic mixture with the water contained in the aqueousacetic acid, distilling water and agent from the mixture, condensing thedistillate, separating distillate into an extracting and withdrawingagent layer and a water layer, returning at least a part of theextracting and withdrawing agent into extractive contact with aqueousacetic acid undergoing concentration, and subjecting the resultantextract mixture of aqueous acetic acid and agent to distillation in thesaid system.

8. A continuous process for concentrating aqueous acetic acid includingdistillation and extraction, which comprises feeding a distillationsystem with an extract comprising aqueous acetic acid and a quantity ofdiethylene dioxide withdrawing and extracting agent, subjecting theextract materials to a distillation treatment which includes controllingthe heating of the distillation system, the rate of supply of aceticacid, and the maintenance of the withdrawing and extracting agent not inexcess of the right amount to form an azeotropic mixture with the watercontained in the acetic acid present in the distillation system,vaporizing a distillate from the distillation system comprising waterand withdrawing and extracting agent, and recovering and returning tothe still at least a part of the withdrawing and extracting agent or thecondensed distillate.

9. A process for concentrating aqueous acetic acid includingdistillation and extraction, which comprises subjecting aqueous aceticacid to at least one extraction treatment with a ketone dehydratingagent having withdrawing and extracting properties and a boiling pointbetween 88 C. and 103 0., subjecting the resultant extract todistillation under conditions wherein the withdrawing and extractingagent is maintained in a quantity not in excess of the right amount toform an azeotropic mixture with the water contained in the distillationsystem, distilling water and dehydrating agent from the system,condensing the distillate, separating the distillate into a layercomprised principally of ketone withdrawing and extracting agent andanother layer comprised principally of water, returning at least aportion of the ketone withdrawing and extracting agent layer to thedistillation step, and withdrawing from the system concentrated aceticacid free of ketone withdrawing and extracting agent.

DONALD F. O'I'HMER.

